360R-06 Design of Slabs-on-Ground
partially in the 30 ft direction before the adjacent strip is
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- A4.3—Design example: Equivalent tensile stress design
partially in the 30 ft direction before the adjacent strip is placed. Final stress will tie all strips together on the end. When calculating the force to overcome the subgrade friction, the total width of all strips is to be considered (12 x 30 = 360 ft). Case 2: A section of 200 ft is placed first, stressed partially, and then the other section of 160 ft is placed and stressed. When calculating the force to overcome the subgrade friction, use the following criteria: Placement 1: Formula W slab × × μ = A Placement 2: = B The tendons in Placement 1 have to overcome maximum friction based on 180 ft length at the critical section at the center of the combined length (dashed line). The tendons in Placement 2 have to overcome maximum friction based on 160 ft length at the critical section at the joint between Placement 1 and 2 (pulling Placement 2 toward Placement 1). A4.3—Design example: Equivalent tensile stress design Determine the reduction in slab thickness of a 6 in. thick unreinforced slab if post-tensioning is used. Assume a modulus of rupture of 9 with a safety factor of 2 was used to design the unreinforced 6 in. thick slab. Then, the allowable tension stress for 4000 psi concrete would be = 285 psi If the P-T force will provide an effective residual compression of 150 psi (selected for this example) with the tendons in the center of the slab, then the allowable tensile stress due to the bending moments is 150 psi + 285 psi = 435 psi. The moment capacity of the slab is given by L 2 --- 6 in. 12 in./ft ------------------- 150 lb/ft 3 500 ft 2 -------------- 0.5 × × × P e f p WH P r + ------------------------- 26,000 lb 250 psi 12 in. 6 in. 9375 lb/ft × × × ------------------------------------------------------------------------------------ P h 2 ----- h 3 ( ) log 1.6a 2 h 2 + 0.675h ) – f c ′ L 2 --- L 2 --- 360 2 --------- 180 ft = = L 2 --- 160 ft = f c ′ 9 4000 psi 2 ---------------------------- |